Identification of molecular and subsurface oxygen on stepped Rh(711)

Abstract

The adsorption of oxygen on Rh(711) was studied over a wide range of oxygen coverages using temperature programmed desorption, high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). Oxygen adsorption on Rh(711), a Rh(100) surface with four atom terraces and a (111) step, differs significantly from the behavior observed on the flat Rh(100) surface. Unlike Rh(100), the Rh(711) surface shows a strong O-O stretch at 1079 cm −1, considerable amounts of subsurface oxygen, and a LEED pattern which indicates a surface reconstruction. Rh(711) adsorbs oxygen dissociatively at 100 K and at low coverages, gives a single HREELS Rh-O mode which varies from about 410 to 435 cm −1 with increasing coverage, and a sharp c(2 × 2) LEED pattern. Desorption from this state grows in with the peak temperature shifting from 1250 to 1135 K characteristic of second-order desorption kinetics. We conclude that oxygen atoms adsorb initially on the terraces, rather than the steps, on this surface. At higher coverages, a new first-order or quasi-zero-order peak appears at 835 K at the same time that desorption from a molecular state is observed in two peaks at 125 and 150 K. On the fully saturated surface, the O-O stretch is observed at 1079 cm −1 which we assign to a superoxo molecular oxygen species. At high oxygen coverages, complicated LEED patterns are observed that suggest a reconstruction of the clean Rh(711) surface structure. O/Rh AES peak intensity ratios show that oxygen goes subsurface at these high coverages and HREELS shows two atomic oxygen stretches at 413 and 534 cm −1, assigned to oxygen in four- and three-fold sites, respectively. The results show that the four-atom wide Rh(100) terraces are different from flat Rh(100) single crystals with respect to: atomic oxygen ordered structures, facile subsurface oxygen diffusion, and HREELS detection of molecular oxygen.